| Modelling of supercritical solid bed batch extraction has been carried out as a function of solid material properties. In case of continuous extraction within an extruder the solid is compacted at the inlet in order to form a gas tight plug. This mechanical treatment takes influence on the solid characteristics within the following extraction zone like the size of solid agglomerates which on its turn determines extraction efficiency. Further little is known about possible solvent loading. Whether or not experience from batch extraction may be applied depends on distribution of the solid within the extraction zone and phenomena like channelling. A simplified model based on the idea of a cascade of ideally mixed tanks was developed in order to describe the continuous counter current process. Figure 41 shows the influence of CO2-flow at constant feed flow of 5 kg/h for hop extraction. Figure 42 shows the influence of particle size at the same feed flow and a solvent flow of 10 kg/h. The number of stirred tanks was fixed to 3 i.e. that non-ideal behaviour such as axial dispersion is considered even though at this moment it could not be accurately quantified yet.
Figure 41: Influence of solvent flow on extraction performance
Figure 42: Influence of particle size on extraction performance
Figure 41 shows a typical curve of a counter current contactor as also known from heat transfer. Here, the de-oiling of solid is depicted as a function of the axial position in terms of theoretical stages. At low CO2 – flow entering the contactor from the right hand side, solvent capacity is limiting for the de-oiling process. Thus the solid being retrieved (at the right hand side) still has a relatively high oil content. A critical solvent mass flow exists, at which the concentration curve is bent to the opposite side being the available oil the limiting factor for extraction arriving at relatively low rest oil content within less stages.
In an analogous way, particle size takes influence on the extraction performance (Figure 42): in case of small particles the solvent is saturated quickly with no net mass transfer happening in the last stage whereas large particles lead to high mass transfer resistance requiring the complete extraction zone for sufficient de-oiling to take place.
To conclude, the fixed bed properties compressibility, permeability and pressure propagation for natural material (hop, caraway, cocoa) were examined with respect to supercritical batch and continuous extraction processes. Extraction modelling of continuous supercritical extraction in an extruder-extractor was introduced. In terms of process feasibility the most important processing variables having a huge impact on the mentioned fixed bed properties were found to be moisture and temperature. For batch extraction fine material has to be sufficiently dry in order to prevent from compacting and inefficient extraction. In extrusion extraction however compaction is the principle of sealing the extraction zone, thus a minimum moisture content is required. Temperature has an effect as well, though less pronounced. Adapting temperature and moisture with respect to the desired compaction behaviour however can have disadvantageous effects on extraction kinetics, so in each case an optimum solution has to be found. |